The present invention relates to ceramic composites and, in particular, to high temperature ceramic composites containing monazites or xenotimes as a component.
State of the art thermal protection (i.e., heat shields) for space re-entry vehicles such as the space shuttle consists of rigid ceramic tiles attached to the vehicle surface. Each tile is relatively small in area to avoid detachment driven by thermal and mechanical loads. They are costly to fabricate and to bond to the surface. Upon re-entry, tiles are often damaged, which results in a labor intensive, time consuming replacement operation. With the goals of low cost and quick turnaround for reuse of spacecraft, rigid tiles are not desirable.
Flexible insulating blankets made from ceramic fibers show promise for allowing larger component sizes, easier attachment, and damage tolerance. However, they are now used only in areas of the vehicle surface that experience relatively low temperatures because traditional blankets made entirely from silica quickly deteriorate upon exposure to high temperatures. Blankets used presently consist of a relatively thick layer of flexible insulating batting produced from random discontinuous fibers, sandwiched between two layers of thin woven fabric produced from continuous fibers. A silica coating is usually applied to the outer surface to improve the resistance of the surface to damage. The coating infiltrates into the woven fabric face sheet and stiffens it, similar to the effect of starch in textile fabrics. The blankets are initially flexible and are relatively easy to apply to the vehicle surface. However, they rapidly become brittle if exposed to temperatures that are sufficiently high to allow the touching fibers to fuse strongly together or to allow the coating to bond strongly to the fibers. Embrittlement makes the blankets susceptible to damage and forces their replacement. These silica blankets have a reuse temperature of only about 1200xc2x0 F.
Blankets constructed from more refractory fibers (based on alumina or mullite) have also been tried, but they have achieved limited success because the silica coating bonds to the fibers at high temperature causing embrittlement.
To allow significantly expanded replacement of tiles with blankets, coated blankets must be able to withstand reentry temperatures of at least 1800-2300xc2x0 F., while remaining tough and flexible. Therefore, a coating solution for the blanket fibers must retain the flexibility of the fabric while avoiding embrittlement upon exposures to temperatures in this range.
To avoid embrittlement, the coating material must not sinter and bond strongly to the ceramic fibers or react with the fibers at these temperatures. Moreover, the coating material must be stable in oxidizing environments. Rare-earth phosphates (monazite and xenotime) were shown to satisfy these requirements as disclosed in U.S. Pat. Nos. 5,514,474 and 5,958,583, hereby incorporated by reference. The use of these compounds to form oxide composites with weakly bonded fiber-matrix interfaces is described. A method for infiltrating the matrix into a fiber preform using a slurry consisting of alumina powder and solution precursors for the rare-earth phosphate (La-monazite) is also described. This method provides a matrix that is converted during heat treatment to a mixture of the rare-earth phosphate and alumina. The solution precursor does not degrade the properties of composites containing high purity alumina fibers (example, NEXTEL(copyright) 610, manufactured by 3M Company) during this precursor conversion step and subsequent heat treatment to at least 2200xc2x0 F. However, the properties of composites containing other, less refractory, ceramic fibers that are desirable for use in thermal protection systems (example, NEXTEL(copyright) 440, 3M Company) were degraded at temperatures of 2000xc2x0 F. Davis, J. B. et al, Composites: Part A 30, 483-488 (1999).
The present invention provides monazite- or xenotime-based coatings that can stiffen ceramic fabrics without causing embrittlement at temperatures of at least as high as 2300xc2x0 F. In one embodiment the coatings comprise high purity monazite powders. The high purity monazite powders provide a superior coating for use at temperatures above 1800xc2x0 F. In another embodiment, the coatings further include additives, such as SiC, to increase emissivity.
The present invention also provides methods for the synthesis of high purity, stoichiometric monazite powders for use in blanket coatings. In one embodiment the methods comprise forming crystalline particles of the hydrated form of monazite (the mineral rhabdophane) by precipitation from an aqueous solution. The rhabdophane precipitate is then washed in two steps: the first with water and the second with a strong organic base. The water washing step removes unreacted water-soluble species and the strong base wash removes excess phosphorous both of which can degrade the thermal stability of ceramic fibers coated with monazite formulations and heated to temperatures above 2000xc2x0 F.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims.